The prior art includes many processes which employ resins, glue, or other like binder materials to bond cellulosic fibers, to form solid composite sheets. Other systems, for example, such as provided in U.S. Pat. No. 1,631,173 to Weiss, relate to wallboards, including a binder consisting substantially of lignocellulose gelatinized by mechanical disintegration. The gelatinized lignocellulosic binder is preferably disintegrated in the presence of water and caustic soda. Another cellulose composition is described in British Pat. No. 892,744 to Durel, Incorporated. The article produced is made by heating in an essentially dry mixture comprising comminuted lignocellulose and a gaseous agent under superatmospheric pressure.
The literature also describes processes employing materials such as hydrogen peroxide in bleaching whole-wood fibers. For example, patents such as U.S. Pat. No. 2,872,280 to Kindron; 3,023,140 to Textor; and 3,069,309 to Fennell, describe respective bleaching processes using hydrogen peroxide, in caustic medium, the methods of both Fennell and Textor being conducted in at least one disc refiner. The Fennell process also includes the use of sulfuric acid to neutralize a portion of the caustic present. However, the subject method is conducted under alkaline conditions.
U.S. Pat. No. 3,354,825 to Goring is directed to a two-step oxidation process for bonding, for example, the surface of strips of low yield, delignified cellulose pulp fibers. The first step of the process includes oxidizing the surfaces by treatment in a substantially dry state with ozone. Preferably, the ozonation step is conducted, on the basic side, in the presence of gaseous ammonia. The second step includes pressing together the above treated cellulose surfaces, in the presence of moisture, and drying the pressed surfaces to promote surface bonding. A number of problems are inherent in the Goring process. Controling the relative homogeneity of the oxidized cellulose is difficult since regulating the relative gas phase contact between the ozone and the fibers, respectively, is quite an intricate task. It is also a problem to determine the quantitative amount of oxidation actually occurring on the treated surface. Furthermore, from a qualitative standpoint, reactivity is a problem due to relatively poor control of the fiber-gas phase contact zone, which can result in uneven oxidative distribution. However, if one were to increase the amount of gaseous ozone charged to the contact zone in an effort to promote higher quantitative and qualitative oxidation, the result could probably be undesirable degradation of the cellulose structure and an increase in the overall cost of materials. Another drawback to the Goring process is the question of overall oxidation of the sheet. If, for example, the sheets employed were of sufficient thickness, surface treatment of the sheet followed by the application of heat and pressure would not effectively bond the entire sheet structure but, instead, would result in an uneven treatment thereof, i.e., an overtreatment of the surface and undertreatment of the inner core (stratified bonding).
Finally, Stofko et al., of the University of California, Forest Products Laboratory, have described a process for forming bonded whole-wood products, such as plywood and particle board, by joining the respective surfaces of the whole-wood board, or pieces thereof, by topichemical oxidative treatment of these surfaces employing a liquid carrier containing an oxidant, followed by pressing the surface-modified whole-wood under heat and pressure. The process specifically includes spraying the respective whole-wood surfaces with a solution of hydrogen peroxide and sulfuric acid (with or without a catalyst), followed immediately by hot-pressing of the respective bondable surfaces. The problems outlined in connection with the Goring patent are again manifested in the Stofko et al. process with respect to homogeneity, quantitative amount of oxidation, qualtitative oxidative distribution, degradation of the sheet on aging, and, finally, the problem of stratified bonding.